The Geomorphology, Color, and Thermal Properties of Ryugu: Implications for Parent-Body Processes S

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◥ Multicolor optical observations revealed RESEARCH ARTICLE SUMMARY that Ryugu possesses the average spectrum of a Cb-type asteroid and lacks a ubiquitous 0.7-µm absorption band. These spectral obser- ASTEROIDS vations and a principal components analysis suggest that Ryugu originates from the Eulalia or Polana asteroid family in the inner main Thegeomorphology,color,and belt,possiblyviamorethanonegenerationof parent bodies. thermal properties of Ryugu: Ryugu’s geometric albedo at 0.55 µm is 4.5 ± 0.2%, among the lowest in the Solar Implications for parent-body processes System. Moderately dehydrated carbonaceous chondrites and interplanetary dust particles

S. Sugita*, R. Honda, T. Morota, S. Kameda, H. Sawada, E. Tatsumi, M. Yamada, ◥ (IDPs) are the only me- ON OUR WEBSITE C. Honda, Y. Yokota, T. Kouyama, N. Sakatani, K. Ogawa, H. Suzuki, T. Okada, teoritic samples with sim- N. Namiki, S. Tanaka, Y. Iijima, K. Yoshioka, M. Hayakawa, Y. Cho, M. Matsuoka, Read the full article ilarly low albedos. The N. Hirata, N. Hirata, H. Miyamoto, D. Domingue, M. Hirabayashi, T. Nakamura, at http://dx.doi. high boulder abundance T. Hiroi, T. Michikami, P. Michel, R.-L. Ballouz, O. S. Barnouin, C. M. Ernst, org/10.1126/ and the spectral proper- science.aaw0422 ties of the boulders are S. E. Schröder, H. Kikuchi, R. Hemmi, G. Komatsu, T. Fukuhara, M. Taguchi, T. Arai, ...... H. Senshu, H. Demura, Y. Ogawa, Y. Shimaki, T. Sekiguchi, T. G. Müller, consistent with dehydrated Downloaded from A. Hagermann, T. Mizuno, H. Noda, K. Matsumoto, R. Yamada, Y. Ishihara, H. Ikeda, surface materials, which might be analogous H. Araki, K. Yamamoto, S. Abe, F. Yoshida, A. Higuchi, S. Sasaki, S. Oshigami, to thermally metamorphosed meteorites. ’ S. Tsuruta, K. Asari, S. Tazawa, M. Shizugami, J. Kimura, T. Otsubo, H. Yabuta, The spectra of Ryugu s surfaces occupy a S. Hasegawa, M. Ishiguro, S. Tachibana, E. Palmer, R. Gaskell, L. Le Corre, small area in the dehydration track of our R. Jaumann, K. Otto, N. Schmitz, P. A. Abell, M. A. Barucci, M. E. Zolensky, F. Vilas, principal component space, suggesting that a ’ F. Thuillet, C. Sugimoto, N. Takaki, Y. Suzuki, H. Kamiyoshihara, M. Okada, K. Nagata, largevolumeofRyugusoriginalparentbody M. Fujimoto, M. Yoshikawa, Y. Yamamoto, K. Shirai, R. Noguchi, N. Ogawa, F. Terui, experienced similar degrees of partial dehy- http://science.sciencemag.org/ S. Kikuchi, T. Yamaguchi, Y. Oki, Y. Takao, H. Takeuchi, G. Ono, Y. Mimasu, dration. Such uniformity is more consistent K. Yoshikawa, T. Takahashi, Y. Takei, A. Fujii, C. Hirose, S. Nakazawa, S. Hosoda, with internal heating on the parent body than O. Mori, T. Shimada, S. Soldini, T. Iwata, M. Abe, H. Yano, R. Tsukizaki, M. Ozaki, heating due to multiple impacts. Nevertheless, K. Nishiyama, T. Saiki, S. Watanabe, Y. Tsuda it is possible that global partial dehydration could result from impacts if the parent body sustained many impacts before its catastrophic INTRODUCTION: The asteroid 162173 Ryugu disruption. Geochemical analyses of thermally is the target of the Japanese Hayabusa2 metamorphosed meteorites are consistent with mission, which is designed to collect samples short-term heating; thus, this scenario cannot from Ryugu’s surface and return them to be readily discarded. on April 25, 2019 Earth. We seek to understand Ryugu’sfor- A third possibility is that Ryugu is covered mation from a parent body, both to better with materials that experienced only incipient explain the origin of near-Earth asteroids and aqueous alteration, possibly similar to some to provide context for analyzing the samples. IDPs. If so, the spectral trend observed in Theoretical calculations indicate that Ryugu- Ryugu’s boulders may be a progression of size asteroids are likely produced through cata- aqueous alteration. strophic disruption of a parent body, formed in the early Solar System, whose fragments then CONCLUSION: Multiple scenarios remain vi- reaccumulated. Ryugu later migrated from the able, but the Hayabusa2 remote-sensing data main asteroid belt to its current near-Earth orbit. are most consistent with parent-body partial dehydration due to internal heating. This RATIONALE: Hayabusa2 rendezvoused with scenario suggests that asteroids formed from theasteroidinJune2018.Detailedglobal ≤ Hayabusa2’s shadow on the surface of materials that condensed at 150 K (the H2O observations of Ryugu were conducted with asteroid Ryugu. The shadow of the solar condensation temperature under typical solar Hayabusa2’s remote-sensing instruments, in- panels spans 6 m. The bright halo is due to nebula conditions) musthaveeitherformed cluding the optical navigation cameras (ONCs), the opposition effect, which enhances the sufficiently early to contain high concentrations laser altimeter [light detection and ranging 26 reflectance at small solar phase angles. of radiogenic species, such as Al, or formed (LIDAR) altimeter], and a thermal infrared near the Sun, where they experienced other camera (TIR). We examined the asteroid’s We estimate that the impact craters pene- heating mechanisms. The degree of internal surface colors, geomorphological features, and trating the top 10 meters of Ryugu’s surface heating would constrain the location and/or thermal properties to constrain models of its have existed for 107 to 108 years, indicating that timing of the snow line (the dividing line be- formation. the last major resurfacing likely occurred while tween H2O condensation and evaporation) in Ryugu was still located in the main asteroid the early Solar System. ▪ RESULTS: Geologic features on Ryugu include belt. In contrast, the low number density of a circum-equatorial ridge, an underlying east- small craters (~10 m in diameter) suggests a The list of author affiliations is available in the full article online. ð≲ 6 Þ *Corresponding author: Email: [email protected] west dichotomy, high boulder abundance, im- very young resurfacing age 10 years for the Cite this article as S. Sugita et al., Science 364, eaaw0422 pact craters, and large-scale color uniformity. top 1-meter layer. (2019). DOI: 10.1126/science.aaw0422

Sugita et al., Science 364, 252 (2019) 19 April 2019 1of1 RESEARCH

◥ its surface. Detailed global observations of Ryugu RESEARCH ARTICLE were conducted with Hayabusa2’s remote-sensing instruments, including the optical navigation cameras (ONCs), one of which is the nadir-viewing ASTEROIDS telescopic camera (ONC-T), with seven narrow- band filters [0.40 mm (ul), 0.48 mm (b), 0.55 mm (v), 0.59 mm (Na), 0.70 mm (w), 0.86 mm (x), and Thegeomorphology,color,and 0.95 mm (p)] (3–5); a laser altimeter [light detection and ranging (LIDAR) altimeter] (6); and a thermal infrared camera (TIR) (7), sensitive to thermal properties of Ryugu: wavelengths from 8 to 12 mm(8). Global images were obtained from the home Implications for parent-body processes position located 20 km above the asteroid (9), from which we constructed a 0.55-mm map of S. Sugita1,2*, R. Honda3, T. Morota4, S. Kameda5, H. Sawada6, E. Tatsumi1, Ryugu (Fig. 1A). Major geomorphologic fea- M. Yamada2, C. Honda7, Y. Yokota6,3, T. Kouyama8, N. Sakatani6, K. Ogawa9, tures visible in this map include impact craters, H. Suzuki10, T. Okada6,1, N. Namiki11,12, S. Tanaka6,12, Y. Iijima6†, K. Yoshioka1, boulders, troughs, and an equatorial ridge (Fig. M. Hayakawa6, Y. Cho1, M. Matsuoka6, N. Hirata7, N. Hirata9, H. Miyamoto1, 1B and table S3). D. Domingue13, M. Hirabayashi14, T. Nakamura15, T. Hiroi16, T. Michikami17, Impact craters P. Michel18, R.-L. Ballouz6,19, O. S. Barnouin20, C. M. Ernst20, S. E. Schröder21, H. Kikuchi1, R. Hemmi1, G. Komatsu22,2, T. Fukuhara5, M. Taguchi5, T. Arai23, Impact crater morphologies, including rim and Downloaded from H. Senshu2, H. Demura7, Y. Ogawa7, Y. Shimaki6, T. Sekiguchi24, T. G. Müller25, floor characteristics, provide indicators of surface A. Hagermann26, T. Mizuno6, H. Noda11, K. Matsumoto11,12, R. Yamada7, Y. Ishihara6‡, age and mechanical properties. Approximately 30 circular depressions ≥20 m in diameter have been H. Ikeda27, H. Araki11, K. Yamamoto11, S. Abe28, F. Yoshida2, A. Higuchi11, S. Sasaki29, identified on Ryugu, many (at least half) with S. Oshigami11, S. Tsuruta11, K. Asari11, S. Tazawa11, M. Shizugami11, J. Kimura29, raised rims (Fig. 1A). Several craters also exhibit T. Otsubo30, H. Yabuta31, S. Hasegawa6, M. Ishiguro32, S. Tachibana1, E. Palmer13, bowl-like shapes (Fig. 2, A and B), whereas others R. Gaskell13, L. Le Corre13, R. Jaumann21, K. Otto21, N. Schmitz21, P. A. Abell33, 34 33 13 18 1 1 have shallow floors (fig. S10). The bowl-shaped http://science.sciencemag.org/ M. A. Barucci , M. E. Zolensky , F. Vilas , F. Thuillet , C. Sugimoto , N. Takaki , depressions are classified on the basis of rim mor- 1 1 1 8 6 6,12 Y. Suzuki , H. Kamiyoshihara , M. Okada , K. Nagata , M. Fujimoto , M. Yoshikawa , phology and shape, providing confidence levels 6,12 6 6 6 6 6 Y. Yamamoto , K. Shirai , R. Noguchi , N. Ogawa , F. Terui , S. Kikuchi , (CLs) to their identification as impact craters. CL1 6 1 1 6 27 6 T. Yamaguchi §, Y. Oki , Y. Takao , H. Takeuchi , G. Ono , Y. Mimasu , features are circular with a clearly identifiable 27 6 6,27 6 27 6 K. Yoshikawa , T. Takahashi , Y. Takei , A. Fujii , C. Hirose , S. Nakazawa , rim, CL2 depressions are circular but exhibit no S. Hosoda6, O. Mori6, T. Shimada6, S. Soldini6, T. Iwata6,12, M. Abe6,12, H. Yano6,12, rim, CL3 depressions are quasi-circular, and CL4 R. Tsukizaki6, M. Ozaki6,12, K. Nishiyama6, T. Saiki6, S. Watanabe4,6, Y. Tsuda6,12 features are circular patterns of boulders with no clear topography. CL1 and CL2 depressions are The near-Earth carbonaceous asteroid 162173 Ryugu is thought to have been produced most likely impact craters. The group of CL3 and from a parent body that contained water ice and organic molecules. The Hayabusa2 CL4 features may include a few craters. Different

spacecraft has obtained global multicolor images of Ryugu. Geomorphological features levels of confidence are used in the statistical on April 25, 2019 present include a circum-equatorial ridge, east-west dichotomy, high boulder abundances analyses to examine the robustness of the results. across the entire surface, and impact craters. Age estimates from the craters indicate Laser-altimeter measurements indicate that fresh a resurfacing age of ≲ 106 years for the top 1-meter layer. Ryugu is among the darkest bowl-shaped depressions have depth/diameter known bodies in the Solar System. The high abundance and spectral properties of ratios ranging from 0.14 to 0.2 (Fig. 2, C and D) boulders are consistent with moderately dehydrated materials, analogous to thermally (8). Although recent numerical calculations (10) metamorphosed meteorites found on Earth. The general uniformity in color across Ryugu’s show that large cavities may be formed in fast- surface supports partial dehydration due to internal heating of the asteroid’s parent body. spinning asteroids via the release of large boulders due to centrifugal force, raised rims are not ex- he asteroid 162173 Ryugu is the target of disruption and reaccumulation of fragments pected to be generated in such a process. Thus, the Japan Aerospace Exploration Agency’s during evolution of the Solar System (1, 2). We large equatorial craters on Ryugu are unlikely to (JAXA’s) Hayabusa2 mission, which arrived seek to understand the properties of both Ryugu have been formed by asteroid spin and are most T in June 2018. Small asteroids, such as and its parent body. Deciphering the geologic likely of impact origin. Ryugu, are thought to have been born from record of an asteroid requires identification and Some craters show evidence for motion of a much older parent bodies through catastrophic characterization of the geological features on loosely consolidated mass of materials on the

1The University of Tokyo, Tokyo 113-0033, Japan. 2Planetary Exploration Research Center, Chiba Institute of Technology, Narashino 275-0016, Japan. 3Kochi University, Kochi 780-8520, Japan. 4Nagoya University, Nagoya 464-8601, Japan. 5Rikkyo University, Tokyo 171-8501, Japan. 6Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), Sagamihara 252-5210, Japan. 7University of Aizu, Aizu-Wakamatsu 965-8580, Japan. 8National Institute of Advanced Industrial Science and Technology, Tokyo 135-0064 Japan. 9Kobe University, Kobe 657-8501, Japan. 10Meiji University, Kawasaki 214-8571, Japan. 11National Astronomical Observatory of Japan, Mitaka 181-8588, Japan. 12SOKENDAI (The Graduate University for Advanced Studies), Hayama 240-0193, Japan. 13Planetary Science Institute, Tucson, AZ 85719, USA. 14Auburn University, Auburn, AL 36849, USA. 15Tohoku University, Sendai 980-8578, Japan. 16Brown University, Providence, RI 02912, USA. 17Kindai University, Higashi-Hiroshima 739-2116, Japan. 18Université Côte d’Azur, Observatoire de la Côte d’Azur, Centre National de le Recherche Scientifique (CNRS), Laboratoire Lagrange, 06304 Nice, France. 19University of Arizona, Tucson, AZ 85705, USA. 20Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA. 21German Aerospace Center (DLR), Institute of Planetary Research, 12489 Berlin, Germany. 22International Research School of Planetary Sciences, Università d’Annunzio, 65127 Pescara, Italy. 23Ashikaga University, Ashikaga 326-8558, Japan. 24Hokkaido University of Education, Asahikawa 070-8621, Japan. 25Max-Planck-Institut für Extraterrestrische Physik, 85748 Garching, Germany. 26University of Stirling, FK9 4LA, Scotland, UK. 27Research and Development Directorate, JAXA, Sagamihara 252-5210, Japan. 28Nihon University, Funabashi 274-8501, Japan. 29Osaka University, Toyonaka 560-0043, Japan. 30Hitotsubashi University, Tokyo 186-8601, Japan. 31Hiroshima University, Higashi-Hiroshima 739-8526, Japan. 32Seoul National University, Seoul 08826, Korea. 33NASA Johnson Space Center, Houston, TX 77058, USA. 34Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA)–Observatoire de Paris, Paris Sciences et Lettres (PSL), Centre National de le Recherche Scientifique (CNRS), Sorbonne Université, Université Paris-Diderot, 92195 Meudon Principal Cedex, France. *Corresponding author: Email: [email protected] †Deceased. ‡Present address: National Institute for Environmental Studies, Tsukuba 305-8506, Japan. §Present address: Mitsubishi Electric Corporation, Kamakura 247-8520, Japan.

Sugita et al., Science 364, eaaw0422 (2019) 19 April 2019 1of11 RESEARCH | RESEARCH ARTICLE inner walls (i.e., wall slumping; Fig. 2A). This terrace structure, which would form if there consolidated target, not dominated by pristine phenomenon was not observed on another sub- was a large strength contrast between the sur- mechanical strength of the constituent sur- kilometer asteroid, Itokawa (11), and indicates face and near-surface interior (12). These floor face material. Therefore, both gravity and weak thepresenceofanunconsolidatedsurfacelayer. morphologies and the high fraction of raised cohesion strength controlled the crater for- No crater has been found to have a floor with rims indicate that the craters formed in an un- mation. The individual boulders in the surface Downloaded from http://science.sciencemag.org/

on April 25, 2019

Fig. 1. Global map and images of Ryugu. (A) Geologic map of Ryugu region through the south polar region to the other side of Ryugu, and based on mosaicked v-band (0.55 mm) images. Impact craters and the large and bright Otohime Saxum (red arrow) near the south pole. The crater candidates are indicated with circles, color coded by confidence location of the poles and the spin direction are indicated with white level (see text). There is greater latitudinal exaggeration of map-projected arrows. (C) Asymmetric regolith deposits on imbricated flat boulders surface area on Ryugu than for a sphere, because of its diamond-like on the northern slope of the circum-equatorial ridge of Ryugu cross section. This leads to the apparent higher crater number density in (hyb2_onc_20181003_222509_tvf_l2b). Small yellow arrows at the edges of the equatorial region of this map. (B) Oblique view of Ryugu (image regolith deposits indicate the direction of mass wasting. The large yellow hyb2_onc_20180824_102748_tvf_l2b), showing the circum-equatorial arrow indicates the current geopotential gradient from high to low (17). The ridge (yellow arrows), trough (blue arrows) extending from the equatorial direction of geopotential gradient is consistent with the mass wasting.

Sugita et al., Science 364, eaaw0422 (2019) 19 April 2019 2of11 RESEARCH | RESEARCH ARTICLE

Fig. 2. Craters on Ryugu. (A)The B largest crater, Urashima (290 m in A diameter, 8.3°S, 92.5°E), on Ryugu (hyb2_onc_20180720_071230_tvf_l2b). Wall slumping is indicated with yellow arrows. (B) Kolobok crater (240 m, 1.5°S, 333.5°E), which has a deep floor, bowl-like shape, and a raised rim (hyb2_onc_20180720_100057_tvf_l2b). (C) LIDAR profiles of Urashima crater. Wall slumping is indicated with blue arrows. (D)LIDAR profiles of Kolobok crater. (E)CSFD on Ryugu and Itokawa and empirical saturation and crater production curves (54)with (orange) and without (green) dry-soil C D cohesion. Black crosses in (F) represent Itokawa crater candidates (11). Red and blue points indicate

Ryugu craters with different crater Distance from Downloaded from CLs. (F)AnR-plot(theCSFD asteroid center (m) normalized by D−2, where D is Distance from crater center (m) Distance from crater center (m) diameter) for Ryugu (circles and squares) and Itokawa (crosses). 103 1 R The relative crater frequency is E Ryugu (CL 1-2) F defined as the differential crater Ryugu (CL 1-3)

) empirical saturation -2 200 Ma frequency in a diameter range http://science.sciencemag.org/ 100 10 Ma between D/k and kD,divided 0.1 100 Ma 3 1/4 5 Ma by D , where k is 2 .Saturation 50 Ma 2 Ma and crater production curves 20 Ma 10 1 Ma are the same as in (E). Ma, 0.01 10 Ma (Cohesionless) million years. 8.9 ± 2.5 Ma (Cohesionless) 5 Ma 2 Ma 1 0.001 1 Ma

empirical (Dry-soil cohesion) 158 ± 47 Ma (Dry-soil cohesion) -4

0.1 Relative Crater Frequency (R) 10

saturation on April 25, 2019

Cumulative Crater Frequency (km Ryugu (CL 1-2) Ryugu (CL 1-3) Itokawa 0.01 10-5 1 10 100 1000 104 1 10 100 1000 104 Diameter (m) Diameter (m)

layer are large (~3 m in diameter) and are sim- comes from the influence of a small amount of likely through n6, the innermost resonance zone ilar to the sizes of the projectiles (~0.1 to 1 m) cohesion, which controls the excavation flow near of the main belt (9, 15). Because the collision that formed craters (~1 to 30 m) on Ryugu (8). the crater rim. On a high-gravity planet, such as frequency is far greater in the main belt, the Therefore, these large boulders may reduce crater Earth, small cohesive forces do not influence crater majority of the craters ≥100 m probably formed size via armoring, in which a large fraction of size, but on microgravity bodies, such as Ryugu, while Ryugu was still resident in the main as- impact energy is lost into the crushing or crater- crater size is influenced by the presence or absence teroid belt. ing of the first-contact boulder instead of forming of small cohesive forces, which is difficult to sim- Ryugu’s CSFD shows a dearth of small craters a granular crater on the asteroid (13). Impact ex- ulate in laboratory experiments on Earth. Despite (<100 m; Fig. 2F). The deficit of small craters on periments using targets with constituent boulder these uncertainties, the observed crater size- Ryugu as a function of crater size is similar to sizes similar to the projectile’s size indicate that frequency distribution (CSFD) indicates that the that found for both Itokawa and Eros (16). Be- armoring effects influence crater size and that a surface age of Ryugu is equal to or younger than cause craters in this size range on Ryugu are not scaling relation for such targets must include a the collisional lifetime (the mean time for an greatly influenced by crater size reduction [owing term for the breakup energy of the first-contact asteroid to experience a collision-induced cata- to armoring effects (14)], these depletion patterns target boulder (14). strophic disruption) of kilometer-sized objects in are more likely to be due to crater erasure pro- We estimated the surface crater retention age the main asteroid belt [(3 to 5) × 108 years (1, 2)] cesses, such as seismic shaking (16). The reduced on the basis of the number density of craters 100 and equal to or older than the expected time (~4 × number of small craters (<100 m) indicates that to 200 m in diameter, using a scaling rule with 107 years) Ryugu has stayed in a near-Earth orbit the crater retention time in this size range is very armoring effect (14), both with and without a (9). Different collision frequencies must be con- brief. The number density of craters ~10 m in- dry-soil cohesion factor, obtaining surface ages of sidered for different scales of surface ages, be- dicates that the average resurfacing of the top 108 and 107 years, respectively (Fig. 2E) (8). This cause Ryugu is thought to have migrated from ~1-meter layer on Ryugu is <106 years for the large (i.e., a factor of 10) uncertainty in surface age the main belt to its current near-Earth orbit, most main-belt impact flux (Fig. 2F) and <2 × 106 years

Sugita et al., Science 364, eaaw0422 (2019) 19 April 2019 3of11 RESEARCH | RESEARCH ARTICLE for near-Earth impact flux (fig. S3). Because im- the different rotational phases observed (Fig. 3A). (19, 31). We derived the disk-integrated surface pact cratering is a stochastic process, some of these The color properties are consistent with the reflectance at the standard laboratory observa- small craters must have formed more recently, classification of Ryugu as a Cb-type asteroid, on tion angles (i, e, a =30°,0°,30°;wherei, e,anda exposing fresh material. the basis of the Bus taxonomy (19). This spectral are incident, emission, and solar phase angles, type establishes connections with potential main- respectively) for comparison with meteorite sam- Mass wasting belt asteroid families and Ruygu’s parent body. ples measured in the laboratory. There is abundant evidence for mass movement Some ground-based spectral observations have The average value of the standard-condition along slopes (i.e., mass wasting) on Ryugu, par- suggested the presence of hydrated minerals, reflectance factor is 1.88 ± 0.17% at 0.55 mm, ticularly around the equatorial ridge and several owing to spectral features observed at 0.7 mm which is lower than that of any meteorites re- craters, such as Urashima crater (Fig. 2A). Some and <0.55 mm(20–22). This would imply that ported in the literature. The darkest meteorite groups of boulders observed along the equatorial Ryugu’s parent body could be a Ch-Cgh asteroid, samples described in the published literature are ridge overlap one another with preferred ori- similar to those in the Erigone asteroid fam- the thermally metamorphosed CCs (26, 32). Re- entations (i.e., imbrication) away from the ridge, ily (23). However, the ONC-T color observations cent measurements show that meteorites of this andtheyareaccompaniedbyasymmetricallydis- rule out such a parent body, as do results from type (e.g., Jbilet Winselwan, Y-86029, and Y- tributed fragmental debris deposits (i.e., regolith). Hayabusa2’s Near-Infrared Spectrometer (NIRS3), 793321) exhibit reflectance similar to that of The edges of these boulders display little to no which show that Ryugu’sgloballyaveragednear- Ryugu (Fig. 3B). These meteorite samples are regolith deposits along the downhill sides of the infrared spectrum does not have any strong OH classified in the weakly heated and moderately ridge (Fig. 1C). Such imbrication typically occurs absorption band signature around 2.8 mm; only heated groups (stages II and III, respectively), in during landslides. The asymmetric regolith de- aweakabsorptionbandisseenat2.72mm(24). which hydrated silicates have been altered into posits along these boulders indicate that the The observed visible spectral type is close to amorphous silicates as a result of dehydration direction of recent mass wasting is from the top that of the asteroids Eulalia and Polana, which but have not recrystallized into olivine or pyroxene of the equatorial ridge toward higher latitudes, aretheparentbodiesofC-complexasteroid (33). Most of these samples are powders; rough Downloaded from consistent with Ryugu’scurrentgeopotential families in the inner main belt (25). The asteroid slab surfaces, such as Ryugu’s surface, generally (9, 17). Wall slumping is observed along crater Erigone has a different spectral type (Fig. 3A). exhibit lower reflectance and bluer spectra. Al- walls, as discussed above (e.g., Fig. 2A). Some Orbital dynamics calculations have shown that though spectral data for slab samples are not as craters, such as Momotaro (12.5°N, 51.9°E) near the most likely origin of Ryugu is either Eulalia frequently reported as data for powder samples, the equatorial ridge, exhibit a higher concentra- or Polana (15). The collisional lifetime of Ryugu slab spectra of major CCs from each clan, such 8 tion of large boulders on their floors than on [(3 to 5) × 10 years] is similar to or less than the as Murchison and Mighei for CM and Ivuna for http://science.sciencemag.org/ þ370 34 35 their rims and walls (fig. S8). Such preferential breakup time of these families [830150 million CI, have been measured ( , ). These slab concentration of larger boulders in topographic years and 1400 ± 150 million years for Eulalia and samples are not consistent with Ryugu’sspectrum. lows also occurs as a result of mass wasting. Polana, respectively (15)]. Ryugu might not be These observations suggest that the equatorial composed of material directly ejected from one of Local color variation and – ridge is made of mechanically unconsolidated these large bodies but could be the product of more age color relation materials and may have formed during a period than one generation of disruption (see below). Surface colors at specific locations on Ryugu of rapid spin (17), as material flowed toward an The link between Ryugu and Cb asteroids in span a large range, and all are consistent with equatorial topographic low. An unconsolidated the inner main belt has implications for pos- the colors of C-complex asteroids (Fig. 3). The nature would have allowed Ryugu to reshape, sible meteorite analogs. The fraction of Cb- to color of the majority of Ryugu’s surface is char- perhaps in response to a change in spin rate. B-type asteroids in the main asteroid belt is acteristic of regolith (denoted “typical regolith”;

When Ryugu was established in its current geo- high; fig. S6 indicates that about half of the Fig. 3, C and D). Other colors are found in rela- on April 25, 2019 potential configuration, the ridge experienced C-complex asteroids in the main belt are of tively limited areas and/or geological features mass wasting as some of the unconsolidated types Cb, B, and C, which do not display a clear on Ryugu, such as distinctive boulders and crater materials flowed toward new topographic lows 0.7-mm band. Their populations in the inner bottoms. Thus, we analyze the color of all pixels on at higher latitudes. Furthermore, interior walls main belt, from which the largest fraction of Ryugu with small incidence and emission angles in large craters, such as Urashima, exhibit evi- near-Earth asteroids is derived (2), contain many (i ≤ 40° and e ≤ 40°) and use them to define the dence for mass wasting, such as imbricated large families, such as the Eulalia and Polana regolith color variation across the surface. boulders and run-up deposits (fig. S8B). This families, with these spectral characteristics (23). The general spectral slope from b-band indicates that mass wasting postdates these Therefore, the B-Cb-C population comprises a (0.48 mm) to x-band (0.86 mm) exhibits the large craters, which must have formed after large fraction of the material reaching Earth greatest regional variation; Ryugu’ssurfacehas the equatorial ridge. as meteorites. There are two major candidates bluer spectral slopes at both poles, on the equa- for Ryugu meteorite analogs with sufficiently torial ridge, and in large troughs (Fig. 3E). Both Disk-averaged color low-albedo materials: thermally metamorphosed polar regions and the equatorial ridge are topo- During Hayabusa2’s approach phase (before carbonaceous chondrites (CCs) (26, 27), or inter- graphic highs, which may be subject to gradual arrival and parking at the home position), a planetary dust particles (IDPs). The latter consist erosion, leading to the exposure of fresh sur- series of disk-averaged photometric observations of highly primitive material that has experienced face material. Steep boulder surfaces, which may were acquired in each of the seven ONC filters no (or only weak) water-rock reaction to form have recently experienced erosion due to ther- over 12 different rotational phases. We used hydrated silicates (28, 29). mal fatigue (36) or other processes, tend to have these data to examine the accuracy of the radio- brighter and bluer surfaces (Fig. 3, C and D). In metric calibrations of ONC-T for each filter. Albedo and reflectance contrast, many locations conducive to deposi- Global reflectance values from the ONC-T images Using the point-source and whole-disk observation, such as crater floors, exhibit redder and based on preflight and in-flight calibrations (3–5) tions of Ryugu, we performed Hapke modeling darker colors. These observations suggest that were compared with ground-based photometric to characterize the disk-integrated photometric exposure of Ryugu materials to space leads to and spectral observations at 0.55 mm(18). The phase behavior in all seven ONC-T filters (table their reddening and darkening. However, it is not results were consistent within the uncertainties, S1) (8). On the basis of these photometric mea- clear whether this trend occurs because of space validating that ONC-T is appropriately calibrated surements, we derived a geometric albedo of weathering or other processes, such as coating over all filters (5, 8). 4.5 ± 0.2% at 0.55 mm, similar to albedos of with redder and darker dust (8). Local-scale het- The disk-averaged seven-band color mea- typical comets (30) and the darkest asteroids, erogeneity suggests that the large-scale uni- surements exhibit little variation (<0.5%) over such as 253 Mathilde, another Cb-type asteroid formity may not be due to pristine materials on

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Ryugu’s surface, but instead may be the result of many near-Earth objects (NEOs) experienced large orbital excursions on a time scale of 107 awell-mixedsurface. dehydration during orbital excursions near the years, and the skin depth of solar heating No regolith-covered surface on Ryugu exhibits Sun, which may have contributed to the depletion during Ryugu’s orbital evolution is tens of cen- astrong0.7-mm absorption in the ONC-T color in 0.7-mm absorption in C-complex NEOs (37). timeters (9). This time scale is longer than the data. Dynamical calculations have shown that NEOs with Ryugu-like orbits may experience retention age (<2 × 106 years) of 10-m craters, Downloaded from http://science.sciencemag.org/

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Fig. 3. Multiband colors of Ryugu’s surface. (A) Comparison between Jbilet Winselwan was measured at 30°, 0°, 30° with the spectrometer disk-averaged spectra (lines with squares, normalized at 0.55 mm) for Ryugu system at Tohoku University (57). The rest of meteorite spectra are from at 12 different rotational phases and ground-based observations (lines (58). (C) Reflectance spectra of typical morphologic and color features on without symbols) of Ryugu from (55) (blue) and from (21)(red).Dataare Ryugu. Locations of features (labeled 1 to 6) are shown in (E) and (F) also shown for the large main-belt asteroids Polana, Eulalia, and Erigone and in fig. S12. Individual spectra are shifted vertically for clarity. Vertical- (56), each of which is the parent body of an asteroid family. Because of the axis tick spacing is 0.05%. (D) Same as (C), but normalized by the Ryugu similarity among the spectra taken at different phases, individual lines for average spectrum. Vertical-axis tick spacing is 0.01. (E) b-x slope map Ryugu overlap. Spectra are offset by 0.1 for clarity. (B) Comparison between (inverse micrometers) and (F) v-band reflectance factor map (percent) typical Ryugu surface colors (black) (reflectance factor at 30°,0°, 30°) and superposed on a v-band image map. The equatorial ridge and the western those of dehydrated CCs (blue) and typical CCs (red). Individual meteorite side (160°E to 290°E) have slightly higher v-band reflectances than other names are indicated. The spectrum of a powder sample (≤155 mm) of regions (see fig. S13 for statistical analysis).

Sugita et al., Science 364, eaaw0422 (2019) 19 April 2019 5of11 RESEARCH | RESEARCH ARTICLE which excavate unheated substrate material (crater shorter exposure to the space environment. In centered at 0.39 and 0.95 mm. Because PCA can depths ~1 m). Thus, the lack of a high degree of contrast, the coincidence between high v-band expand spectra into orthogonal basis functions, hydrationonRyuguisunlikelytobeduetosolar reflectance and low boulder abundance suggests PCA often extracts linear combinations of spectra heating during a recent orbital excursion. that this dichotomy may reflect smaller grain with physical or mineralogic meanings as the size in the western hemisphere. The two hemi- principal components (PCs). Nonorthogonal linear East-west dichotomy spheres may have different physical properties, combinations of the second and third PCs pro- Ryugu’s western side (160°E to 290°E), which such as grain size and mobility, which could duced by our analysis (PC2 and PC3) correspond is surrounded by troughs (Fig. 1, A and B), has a be the result of reaccumulation of two large to the 0.7-mm absorption and drop-off in reflec- v-band albedo higher than that of other areas rubble piles with different grain sizes during tance shortward of v-band (Fig. 5B and fig. S5). (Fig. 3F and fig. S13). This western side also has a the reaccumulation stage immediately after the Our results indicate that regolith spectra from lower number density of large boulders (Fig. 4A). catastrophic disruption of the parent body (see ONC-T are consistent with moderately dehydrated The topographic highs and bluish b-x spectral the “Implications for the evolution of Ryugu’s CCs (e.g., Y-86029) and reside both near the edge slopeoftheequatorialridgetransectthetroughs, parent body” section below). of the B-Cb-C population and the dehydration suggesting that the equatorial ridge formed more tracks for CM and CI chondrites (Fig. 5B and recently than the troughs. Although the equa- Principal components analysis fig. S4). torial ridge has depressions around 160°E and We conducted a principal components analysis TIR observations provide constraints on sur- 290°E, the morphologic characteristics of these (PCA) of the ONC-T filter data and the second face grain size, which influences the PCA results features are more consistent with those of im- phase of the Small Main-Belt Asteroid Spectro- of colors on Ryugu. TIR observations indicate pact craters, so we do not consider them to be scopic Survey (SMASSII) observations of C-complex that the peak temperatures of Ryugu’s surface connected to the mass motion that formed the main-belt asteroids by ground-based telescopes. correspond to uniform thermal inertia values trough. The formation of the east-west dichotomy This method has been used widely for asteroid between 200 and 500 J m−2 s−0.5 K−1 (Fig. 6). probably predates the equatorial ridge formation. spectral analysis and has served as a basis for These values are consistent with the disk-averaged Downloaded from However, because there is no difference in b-x spectral type definitions (19, 38). Because most value (150 to 300 J m−2 s−0.5 K−1)estimatedonthe spectral slope between the western side and other reflectance data registered in SMASSII covers basis of prearrival telescope observations (39)and regions on Ryugu, the nature of its enhanced only 0.43 to 0.9 mm, we limited the ONC-T data suggest subcentimeter to 10-cm grains (7, 40); the reflectanceisprobablynottheresultofa to the b to x bands, excluding the ul and p bands fraction of surface area covered with grains ≤1mm

B http://science.sciencemag.org/ A 80 30 m )

70 20-30 m -2

) 100 -2 60

40 10 30

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Boulder number density (km Boulder number 10 1 Cumulative boulder frequency (km Cumulative 0 60 120 180 240 300 360 10 20 40 60 80 100 200 Longitude (deg) Boulder diameter in max dimension (m)

CD E F

Fig. 4. Statistics and morphologies of boulders on Ryugu. (A) Distribution A close-up view of its layered structure is shown in fig. S11E. (E)Atype3 in longitude of boulders with diameters of 20 to 30 m and ≥30 m. bright and mottled boulder (hyb2_onc_20180801_213221_tvf_l2b). (B) Cumulative size distribution of large boulders, compared between (F) The sole type 4 boulder, Otohime Saxum, has concentric (yellow different latitudinal zones. (C) A type 1 boulder, which is dark and rugged arrows) and radial (blue arrows) fractures, consistent with a fracture system (hyb2_onc_20181004_042509_tvf_l2b). A close-up view of its layered generated by an impact (hyb2_onc_20180719_124256_tvf_l2b). In (C) structure is shown in fig. S11D. (D) A type 2 bright boulder with smooth to (F), the brightness of each image is stretched independently. The yellow and surfaces and thin layered structure (hyb2_onc_20181004_012509_tvf_l2b). whitescalebarsare10and100m,respectively.

Sugita et al., Science 364, eaaw0422 (2019) 19 April 2019 6of11 RESEARCH | RESEARCH ARTICLE is very small. Laboratory examination of CC fects of compaction and thin coating of fine that the distribution of Ryugu’ssurfaceismuch powders of different grain sizes demonstrates powers may influence the spectra (41). Compar- narrower than that of the Murchison dehydration that in the visible wavelengths the reflectance ison between the PCA results of Ryugu’ssurface track in the PC space (Fig. 5B), suggesting that and spectral slope do not change markedly for and the dehydration track for heated coarse- Ryugu is dominated by materials that experienced grain sizes larger than ~1 mm, although the ef- grained samples of the Murchison meteorite shows similar degrees of dehydration.

Fig. 5. Colors of surface features on Ryugu. A Colors measured from ONC-T images are compared between areas of regolith (gray-black contour) and the four types of boulders (solid, monotone squares) on Ryugu. The legend applies to both panels. (A) Comparison of v-reflectance factor and b-x slope distribution. The average value of Ryugu’s surface is indicated with a white cross. Contours indicate 95 and 68% of the surface area. (B) Comparison of principal component space (PC2-PC3) and main-belt C-complex asteroids (56) (colored circles), a moderately dehydrated CC [Y-86029, orange diamond Downloaded from (58)], Murchison (CM2) samples with heating [black line (58)] and laser irradiation [light green (59) and gray lines (58)], and heated Ivuna (CI) samples [blue line (58)]. Parent bodies of major asteroid families in the inner main belt, Polana (open blue star), http://science.sciencemag.org/ Eulalia (solid light blue star), and Erigone (open green star), are also shown (56). Images of the four types of boulders are shown in Fig. 4 and fig. S11. Thick black arrows denote locations of end-member spectra (spectra with deep 0.7-mm absorption, flat spectra, and spectra with deep ultraviolet absorption) in this PC space.

AB EF

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CD GH

TI = 200

Temperature (K) Temperature TI = 200 (shifted) TI = 600

260 300 340 380 TI = 600 (shifted)

8 1012141618208 101214161820 Local Time (hr)

Fig. 6. Thermal infrared camera measurement results. (A)Brightness boulder at (20.9°S, 27.8°E) (hyb2_onc_20180801_174157_tvf_l2b). (G)Temper- temperature image taken with TIR at 06:07:11 UTC on 10 July 2018 atureprofileofthelocationindicatedwiththecirclein(E)observedwith (hyb2_tir_20180710_060711_l2). (B to D) The image in (A) compared with TIR at 20 km from the Ryugu center (open circles). Theoretical temperature calculated thermal images by using the structure-from-motion shape profiles for uniform thermal inertias of 200 and 600 J m−2 K−1 s−0.5 are shown model (17), assuming uniform thermal inertia of (B) 50, (C) 200, and (D) with curves. Solid curves are for a horizonal plane that starts to receive solar 500 J m−2 s−0.5 K−1, respectively. (E) An ONC-T image of large boulders light at local time 7.5 hours; dashed curves represent a tilted plane that receives (6.4°S, 148.4°E), taken during low-altitude (5 to 7 km) observations sunlight at later times. The observed data are largely enclosed by the upper (hyb2_onc_20180801_144909_tvf_l2b). Surface area (open circle) not covered envelopes of time-shifted curves for 200 and 600 J m−2 K−1 s−0.5.(H)Same with regolith was chosen for temperature analysis. (F)Asin(E),butfora as (G), but for the location indicated by the circle in (F).

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Abundance and distribution of boulders average color (Fig. 3). (ii) Type 3: Bright and have a large PC2 change), they differ in PC3 Ryugu’s surface contains many boulders that span mottled boulders. This boulder type does not change, leading to a very different slope in the a wide range of sizes (Fig. 4B). The largest boul- show clear layers but displays a blocky variega- PC2-PC3 space. Instead, the trend formed by ders (>20 m) are too large to be ejecta from the tion in albedo (Fig. 4E and fig. S11C). The bright boulder types 1 to 3 connects the average Ryugu observed craters (≤300 m) (42), which suggests parts exhibit a drop-off in reflectance at short spectrum and the Ch-Cgh population. A simple that they are instead fragments of Ryugu’s par- wavelengths (ul and b) (Fig. 3C). (iv) Type 4: The interpretation of these trends is the mixing be- ent body. The cumulative distribution of boulder largest boulder on Ryugu, Otohime Saxum, does tween two components in Ch-Cgh and B-Cb-C sizes follows a power law with an exponent be- not match the other types. It is located near the populations. This trend is consistent with mix- tween −2.5 and −3 (Fig. 4B), similar to that mea- south pole (Figs. 1 and 4F), with sharp edges and ing seen not only in the regolith but also in the sured for other small asteroids (43–45). smooth surfaces but no obvious layering. Its boulders. This observation based on PCA suggests The global average number density of boulders vertical face is the brightest surface on Ryugu that these boulders have experienced mixing ≥20 m in diameter at latitudes ≤70° is 50 km−2 and exhibits a very blue color (Fig. 3). The dif- processes. on Ryugu, which exceeds the value for Itokawa ferences in brightness among these four types by a factor of 2 (39, 41). However, the number of boulders are not very large; the range of their Boulder texture and regolith grains density of craters on Ryugu is of the same order v-band reflectance factors is similar to that of the Hayabusa2 carried several landers [encompassed of magnitude as that of Itokawa, which suggests background regolith (Fig. 5A). by three MIcro Nano Experimental Robot Ve- that boulders on both asteroids have experienced More quantitative examinations of boulder hicles for Asteroid (MINERVA-IIs) and the Mobile similar degrees of meteoritic bombardment. colors were performed using reflectance-slope Asteroid Surface Scout (MASCOT)]. Deployment of Thus, the higher abundance of large boulders statistics and PCA. Although the dark, rugged these landers required multiple spacecraft descents on Ryugu suggests that the impact strength of boulders and the bright, smooth boulders are to <100-m altitude, which provided opportunities these boulders may be of a similar order of mag- distinct in morphology, they form a single linear for obtaining close-up images down to a scale of nitude to that of Itokawa’s boulders. Given the trend in reflectance-slope diagrams and PC2-PC3 6 mm per pixel (Fig. 7 and fig. S14). Downloaded from apparent high surface mobility, it is possible space (Fig. 5) parallel to the general distribution The higher-resolution images show that the that many previously existing boulders may have of PC scores over the entire surface. The range in global size distribution of both boulders and been ejected from Ryugu as macroscopic bodies. boulder color variation is similar to the color pebbles follows a power-law distribution down If so, fragments from Ryugu may reach Earth as variation seen over the entire surface; the range to decimeter scales; the slope (i.e., power-law macroscopic meteorites. of PC2 scores of 27 large boulders encompasses index) of their cumulative size distribution is ’ – The spatial distribution of boulders on Ryugu the PC2 score range of more than half of Ryugu s about 2.5 at sizes larger than ~0.2 m, similar http://science.sciencemag.org/ differs from that on Itokawa and Eros, which surface. This agreement in PC-score trends be- that found for large blocks (10 to 160 m) (Fig. 4B). have boulder-poor regions, such as smooth ter- tween regolith and boulders suggests that the The slope then becomes shallower at smaller rains (46, 47) and regolith ponds (48). Ryugu color variation in regolith on Ryugu may be con- sizes (fig. S14A). The shallower slope in the small does not contain large areas with low boulder trolled by the color variation of these two types size range suggests a non-negligible mechanical abundance, suggesting that the degree of size of boulders, from which the regolith may be pro- strength of individual boulders and pebbles on sorting is much lower on Ryugu. A contribut- duced through comminution processes. Ryugu. Although such high-resolution measure- ing factor may be the difference in the overall In contrast, the different sides or facets of ments have been conducted only in limited areas shape of these asteroids: Itokawa and Eros are Otohime Saxum form their own trend in the on Ryugu, no differences have been observed be- elongated, whereas Ryugu is spheroidal. How- reflectance-slope diagram (Fig. 5A) and PC spaces tween areas on and off the equatorial ridge. ever, there is evidence for some global size seg- (Figs. 5B and fig. S7), approximately parallel to the Many images of Ryugu exhibit bright spots

regation in the latitudinal variation in boulder trend for heat-induced dehydrated CC materials (Fig. 7, A and B, and fig. S14B). Some of these on April 25, 2019 size: The boulder number density is lower in the (26). The trend observed for Otohime Saxum is spots are brighter than the average background equatorial region than at higher latitudes (Fig. also parallel to the distribution trend in the B-Cb- by a few tens of percent, whereas others are 3A). This may be because of mass flow during C population (Fig. 5B and fig. S4), which suggests brighter by a factor of 2 or greater. These spots the equatorial ridge formation (17). There is also that their color variations may result from the may be impact craters or recent fragments from a smaller boulder abundance variation in the same process, such as dehydration. preexisting boulders, both of which could exhibit longitudinal direction (Fig. 3B), with the boulder In the PC2-PC3 space, the bright, mottled higher albedos because of the freshness of their abundance in the western hemisphere (160°E to boulders are consistent with the Ch-Cgh asteroid interior, or they may be small fragments of dis- 290°E) systematically lower than at all other population, closest to Erigone (Fig. 5B). These tinct intrinsic composition. Their distinctive longitudes on Ryugu. type 3 boulders extend the trend seen in the brightness and relativelylowabundancesuggest regolith and other types of boulders (Fig. 5B that they may be composed of materials similar Color and morphology of boulders and fig. S7). The 0.7-mmabsorption—measured to bright and mottled boulders, whose spectra There is a systematic trend between boulder as the difference, (v + x)/2w – 1, between w-band are consistent with Ch-Cgh populations (Fig. 5). colors and morphologies on Ryugu. We have reflectance and the linear continuum defined by Ryugu’s surface material may be a mixture of identified four distinct morphologic boulder v- and x-band reflectance values—is not stronger materials with different lithologies representa- types. (i) Type 1: Dark and rugged boulders. than the average Ryugu spectrum. Type 3 boulders tive of its parent body. This type possesses rugged surfaces and edges, are close in PC space to the Ch-Cgh population, Heterogeneity in brightness can also be found tends to have uneven layered structures possibly owing to their low b-band reflectance. within individual boulders (Fig. 7A). This mor- related to inclusion of coarse-grained clasts (Figs. The linear trend extending through type 3 phologic characteristic is consistent with coarse- 4C and fig. S11A), and has color properties similar boulders, the average regolith, and type 1 and grained clastic rocks (impact breccia), including to Ryugu’s average color (Fig. 3C). Many boulders 2 boulders is seen in the first three PC values rock fragments broken by impact. The majority of this type are partially buried by regolith; as is plotted against albedo. This trend cannot be constituent of CCs has been proposed to be im- the case for Ejima Saxum (Fig. 1A). (ii) Type 2: produced by space weathering and/or grain size pact breccia (49). This suggests material mixing Bright and smooth boulders. This boulder type effects. It also differs from the L-shaped distribu- before these boulders wereformed,whichlikely displays several thin and parallel layers (Fig. 4D). tion of laboratory dehydration data (Fig. 5B). took place on Ryugu’s parent body. Because im- Many of these boulders are positioned atop the Although the low-temperature evolution of the pact breccias can contain multiple components regolith, and some exhibit distinctive striped dehydration track for CM chondrites is similar with variable mixing ratios, they can readily ac- patterns (fig. S11B). Their typical color ranges to the boulder trend (both cross the dividing gap count for the mixing trends seen in the PC spaces from slightly bluer than average to Ryugu’s between Ch-Cgh and B-Cb-C populations and (fig. S7). Breccia formation on the parent body

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process; efficient heating occurs only around the impact site (fig. S15). Most of the volume does not experience much heating and simply fractures into cold impact fragments. The numerical cal- culation results (50) (fig. S9) indicate that a catastrophic disruption event due to a large impact would sample different portions of the parent body along the excavation streamlines. Thus, any body formed from reaccumulated fragments would be primarily heterogeneous unless the parent body itself was homogeneous—i.e., the large-scale radial heterogeneity in the parent body would be inherited by the boulders com- prising the reaccumulated-fragments body. Con- sequently, the preponderance of materials with little water signature on Ryugu suggests that a dominant part of its original parent body was also water poor. Such global partial dehydration is possible with impacts, but only if many impacts occurred before the catastrophic disruption (Fig. 8). Fig. 7. Close-up observation results of surfaces on Ryugu. (A) A boulder partially buried with Geochemical analyses of thermally metamor- regolith (yellow arrows) and a smaller boulder with angular fragments having different brightness (blue phosed meteorites are consistent with short-term Downloaded from arrow) near the MINERVA-II landing site (9 mm per pixel, hyb2_onc_20180921_040154_tvf_l2b). heating (27, 52); thus, this scenario cannot read- (B) A rugged boulder with layered structure (yellow arrows) near the MASCOT landing site ily be discarded. However, the observation that (6 cm per pixel, hyb2_onc_20181003_003036_tvf_l2b). Ryugu’s regolith and boulders are concentrated in a relatively small area in the dehydration track can also account for porous textures observed IDPs and are inconsistent with completely dehy- in the PC spaces suggests that a large volume of ’ in many dark boulders on Ryugu (Fig. 7B). These drated CCs. In this section, we discuss the origin Ryugu s original parent body was dehydrated http://science.sciencemag.org/ porous boulders seen in the high-resolution im- of these materials on Ryugu. to a similar state. Such uniformity is more con- ages have the same morphologies as the dark, Recent numerical calculations of large asteroid sistent with internal heating on the parent body rugged boulders observed in lower-resolution breakups by collision show that fragments formed than partial dehydration caused by multiple global and regional images (Fig. 4C). These by the reaccumulation of material, resulting in impacts. boulders often have quasi-parallel layers (Fig. 7B). a rubble pile structure, can contain materials An alternative possibility is that Ryugu is Thus, if these boulders are impact breccias, they sampling different depths on the original parent covered with materials that experienced only may originate from the sedimentation of multiple asteroid (~100 km in diameter) (50). Mixtures of incipient aqueous alteration before forming ejecta blankets. impact debris with different lithologies from the Fe-rich serpentine, which has 0.7-mmabsorption. Layered structures are seen on boulder sur- original parent body could deposit on the re- In this scenario, the closest meteoritic counterpart faces, suggesting that these boulders are not accumulated fragments, leading to the formation would be IDPs. If Ryugu is made of such highly

covered with loose regolith, supporting our in- of impact breccias. A subsequent impact on such a primitive materials, the trend connecting rego- on April 25, 2019 terpretation that the thermal inertias of boulders reaccumulated fragment would generate boulders lith and dark boulders in the B-Cb-C population measured by TIR (Fig. 6) reflect the bulk proper- with a large heterogeneity in color properties. with bright, mottled boulders in the Ch-Cgh pop- tiesoftheboulders.Thepresenceofruggedgrains Using a similar method, we conducted numerical ulation may be a progression of aqueous altera- and pores is also consistent with low thermal con- calculations to estimate how much material is tion (28, 29). However, there are insufficient IDP ductivity and density. collected in reaccumulated bodies from different reflectance spectra available to constrain this The porous nature of impact breccias would depths of a 100-km–diameter parent body (8) scenario. It is difficult to distinguish materials increase the bulk porosity of Ryugu. The very (fig. S9). The results indicate that materials from that experienced only a low degree of hydration low bulk density [(1.19 ± 0.02) × 103 kg/m3]of all depths of the parent body are accumulated in from materials that originally were highly hy- Ryuguwouldrequireveryhighporosity(~50%) each small reaccumulated body. This could ac- drated and subsequently experienced partial if the grain densities of typical CCs are assumed count for both the relatively homogeneous spec- dehydration. Nevertheless, the boulders on Ryugu (17). Such a high porosity is substantially greater tral properties of Ryugu and the limited amount have survived impact processes during cata- than that (~40%) for closest packing with a of local heterogeneity found in the boulders, if strophic disruption, the reaccumulation process, single boulder size. If Ryugu possesses pores partial dehydration occurred as a result of internal and more-recent impacts on Ryugu; they are not within individual boulders (intraboulder pores) heating (e.g., due to radioactive decay of 26Al). dust balls with little cohesion. Thus, this scenario in addition to pores between multiple boulders Internal heating can warm a large fraction of the is in conflict with the boulder-rich nature of (interboulder pores), such low density can be volume of the parent body relatively uniformly, Ryugu. If Ryugu is composed of IDP-like ma- achieved with typical CC materials. leaving a small volume of outer layer relatively terials and does not have a macroscopic meteorite cool (51) (Fig. 8). counterpart, there must be an additional mecha- ’ Implications for the evolution of Ryugu s In contrast, partial dehydration due to a nism to break up boulders and pebbles before parent body single-shock heating event, such as that induced they arrive at Earth as meteorites. Our observations suggest the presence of par- by the catastrophic impact that disrupted the Although multiple scenarios for the evolution tially hydrated minerals on Ryugu, though with original parent body, is unlikely because most of Ryugu’s parent body remain viable, our com- a low degree of hydration. The low average albedo boulders on Ryugu do not possess a strong parison between Hayabusa2 remote-sensing data, (Fig. 3B), average spectra lying in the midrange 0.7-mm absorption band. To suppress the 0.7-mm meteoritic samples, and asteroids leads us to of dehydration tracks of CM and CI chondrites bandinthemajorityofaresultingbodycom- prefer the scenario of parent-body partial de- (Fig. 5B), and shortward drop-off in the spectra posed of reaccumulated fragments, the impact hydration due to internal heating. This scenario of some boulders (Fig. 3D) are consistent with must heat the relevant mass to 400°C or higher. suggests that asteroids that accreted materials moderately dehydrated CCs and/or weakly altered However, impact heating is an inefficient global that condensed at ≤150 K (the H2Ocondensation

Sugita et al., Science 364, eaaw0422 (2019) 19 April 2019 9of11 RESEARCH | RESEARCH ARTICLE Downloaded from http://science.sciencemag.org/

Fig. 8. Schematic illustration of Ryugu’s formation. Ryugu formed from the reaccumulation of material ejected from an original parent body by an impact, possibly by way of an intermediate parent body (bottom). Three scenarios to explain Ryugu’s low hydration and thermal processing may have occurred before disruption of the original parent body (top).

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Sugita et al., Science 364, eaaw0422 (2019) 19 April 2019 11 of 11 The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes S. Sugita, R. Honda, T. Morota, S. Kameda, H. Sawada, E. Tatsumi, M. Yamada, C. Honda, Y. Yokota, T. Kouyama, N. Sakatani, K. Ogawa, H. Suzuki, T. Okada, N. Namiki, S. Tanaka, Y. Iijima, K. Yoshioka, M. Hayakawa, Y. Cho, M. Matsuoka, N. Hirata, N. Hirata, H. Miyamoto, D. Domingue, M. Hirabayashi, T. Nakamura, T. Hiroi, T. Michikami, P. Michel, R.-L. Ballouz, O. S. Barnouin, C. M. Ernst, S. E. Schröder, H. Kikuchi, R. Hemmi, G. Komatsu, T. Fukuhara, M. Taguchi, T. Arai, H. Senshu, H. Demura, Y. Ogawa, Y. Shimaki, T. Sekiguchi, T. G. Müller, A. Hagermann, T. Mizuno, H. Noda, K. Matsumoto, R. Yamada, Y. Ishihara, H. Ikeda, H. Araki, K. Yamamoto, S. Abe, F. Yoshida, A. Higuchi, S. Sasaki, S. Oshigami, S. Tsuruta, K. Asari, S. Tazawa, M. Shizugami, J. Kimura, T. Otsubo, H. Yabuta, S. Hasegawa, M. Ishiguro, S. Tachibana, E. Palmer, R. Gaskell, L. Le Corre, R. Jaumann, K. Otto, N. Schmitz, P. A. Abell, M. A. Barucci, M. E. Zolensky, F. Vilas, F. Thuillet, C. Sugimoto, N. Takaki, Y. Suzuki, H. Kamiyoshihara, M. Okada, K. Nagata, M. Fujimoto, M. Yoshikawa, Y. Yamamoto, K. Shirai, R. Noguchi, N. Ogawa, F. Terui, S. Kikuchi, T. Yamaguchi, Y. Oki, Y. Takao, H. Takeuchi, G. Ono, Y. Mimasu, K. Yoshikawa, T. Takahashi, Y. Takei, A. Fujii, C. Hirose, S. Nakazawa, S. Hosoda, O. Mori, T. Shimada, S. Soldini, T. Iwata, M. Abe, H. Yano, R. Tsukizaki, M. Ozaki, K. Nishiyama, T. Saiki, S. Watanabe and Y. Tsuda Downloaded from

Science 364 (6437), eaaw0422. DOI: 10.1126/science.aaw0422originally published online March 19, 2019

Hayabusa2 at the asteroid Ryugu Asteroids fall to Earth in the form of meteorites, but these provide little information about their origins. The http://science.sciencemag.org/ Japanese mission Hayabusa2 is designed to collect samples directly from the surface of an asteroid and return them to Earth for laboratory analysis. Three papers in this issue describe the Hayabusa2 team's study of the near-Earth carbonaceous asteroid 162173 Ryugu, at which the spacecraft arrived in June 2018 (see the Perspective by Wurm). Watanabe et al. measured the asteroid's mass, shape, and density, showing that it is a ''rubble pile'' of loose rocks, formed into a spinning-top shape during a prior period of rapid spin. They also identified suitable landing sites for sample collection. Kitazato et al. used near-infrared spectroscopy to find ubiquitous hydrated minerals on the surface and compared Ryugu with known types of carbonaceous meteorite. Sugita et al. describe Ryugu's geological features and surface colors and combined results from all three papers to constrain the asteroid's formation process. Ryugu probably formed by reaccumulation of rubble ejected by impact from a larger asteroid. These results provide necessary context to understand the samples collected by Hayabusa2, which are expected to arrive on Earth in December 2020. Science, this issue p. 268, p. 272, p. eaaw0422; see also p. 230

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